1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the zoom lens. More specifically, the present invention relates to a zoom lens useful as a photographic optical system used in an image pickup apparatus, such as a digital camera, a video camera, a silver-halide film camera, or a television (TV) camera, and an image pickup apparatus having the zoom lens.
2. Related Background Art
A conventional negative-lead type zoom lens includes a lens unit having negative refractive power provided at a location closest to the object side. This conventional configuration allows the negative-lead type zoom lens to achieve a wide angle of view and a long back focus. Accordingly, in an image pickup apparatus, a negative-lead type zoom lens is often used as a photographic lens having a wide angle of view.
An example of a negative-lead type zoom lens is described in U.S. Pat. No. 4,730,908. U.S. Pat. No. 4,730,908 discusses a five-unit zoom lens, which is constituted by five lens units including, in order from the object side to the image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a negative refractive power, a fourth lens unit having a positive refractive power, and a fifth lens unit having a positive refractive power. In the zoom lens discussed in U.S. Pat. No. 4,730,908, the second through the fourth lens units are moved during zooming.
In addition, U.S. Patent Application Publication No. US 2010/0053765 discusses a zoom lens having a wide angle of view which is constituted by, five lens units including, in order from the object side to the image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power. In the zoom lens discussed in U.S. Patent Application Publication No. US 2010/0053765, the second through the fourth lens units are moved during zooming.
A conventional characteristic of the negative-lead type zoom lens is that it has a lens configuration asymmetry to the aperture stop. Accordingly, if a negative-lead type zoom lens is used, it is difficult to correct variation of various aberrations (in particular, curvature of field, distortion, and chromatic aberration of magnification) during zooming. In addition, in this case, it is difficult to achieve a high optical performance for the entire zooming range.
Furthermore, if the angle of view is broadened and the zoom ratio is increased, the effective diameter of the front lens (i.e., the effective diameter of the first lens unit) is likely to become large. Accordingly, it becomes difficult to reduce the total size of the optical system.
In order to achieve a high optical performance for the entire zooming range in a small-size negative-lead type zoom lens while achieving a wide angle of view, it is desirable to appropriately set the configuration of the lens units, the refractive power assigned to each lens unit, and a condition for moving a magnification-varying lens unit during zooming. In particular, in the negative-lead type five-unit zoom lens described above, it is highly desirable to appropriately set the focal lengths of the first and the second lens units and a condition for moving the second lens unit during zooming.